Photovoltaic systems, called PV systems abbreviated in the text which follows, are used for converting sunlight into electrical energy. For this purpose, a multiplicity of photovoltaic modules, called PV modules abbreviated in the text which follows, each of which represents an interconnection of a number of photovoltaic cells, is usually interconnected electrically as a photovoltaic generator. The photovoltaic generator (PV generator) is connected to an inverter, frequently mounted remotely, which is used for converting the direct current delivered by the PV generator into alternating current which is suitable for being fed into a public or private (isolated operation) power supply system.
In this context, the PV modules are mostly series-connected in such a manner that the direct-current lines running between the PV generator and the inverter are loaded with voltages of substantially more than 100 V. For reasons of efficiency, a voltage of this order of magnitude is suitable, among other things, in order to keep ohmic losses in the lines tolerably small without having to select a line cross section which is too large. With a light incidence on the PV modules, however, the risk of a lethal electric shock exists due to the high voltage in the case of damages, e.g., in the case of fire, or during installation and maintenance work. Without further protective measures, the danger to life in the case of a direct contact or an indirect contact, e.g., via quenching water, can only be banned if the power generation by the PV modules is stopped, for example, by darkening the PV modules. This is difficult to implement in the case of large PV systems or in the case of fire, however.
In order to avoid the occurrence of hazardous voltages especially at accessible or exposed components of a PV-system during certain situations, e.g., in the case of fire or during maintenance work at a PV system, it is known to arrange switching elements, e.g., electromechanical switches, contactors or semiconductor switches in spatial vicinity of the PV modules, for example, in a connecting socket of the PV module. The switching elements are controlled by the inverter or any other control center via control lines and de-energize the power transmitting connecting lines between the PV modules and the inverter. This can be done either by interrupting the connecting lines by means of the switching elements or by short-circuiting the PV modules as disclosed, for example, in the journal Photon, May 2005 edition, pp. 75-77.
The connecting lines between the PV generator and the inverter can also be de-energized by a single switching element arranged at the PV generator as disclosed in the printed document DE 10 2005 018 173 A1. Printed document DE 10 2009 022 508 A1 discloses a similar setup, where a switching element for de-energizing the power transmitting connection lines between a PV generator and an inverter is arranged in close proximity of the PV generator. The switching element is mains-operated for de-energizing the connection lines depending on the status of the buildings power-supply system. In this case, “mains-operated” means that the switching element is directly coupled to the buildings power-supply. Accordingly, a power-supply line has to be provided at the PV-generator.
In all these cases, additional lines have to be provided for transmitting the control signals to the switching elements located at the PV-generator.
As an alternative, it is known from the printed document DE 10 2006 060 815 A1 to send the control signals as radio-frequency signals via the direct-current lines which are used for the transmission of electric power from the PV-generator to the inverter. For this purpose, the switching elements are provided with a control unit which decodes the control signals transmitted at radio frequency and controls the switching process. To generate the radio-frequency control signals, separate and generally elaborate and costly signal generators are provided. The use of radio-frequency control signals also necessitates a relatively high expenditure in the electromagnetic shielding of the signal generators in order to meet the EMC (electromagnetic compatibility) guidelines.